Efficiency improvement in MEMS thermoelectric generators employing solar concentration

Abstract

Thermoelectric generators (TEGs) are devices that convert heat into electricity. The efficiency of thermoelectric generators depends on the temperature difference across the device, the average temperature of operation, and on the thermoelectric properties of the material. Most work on improving the TEG efficiency deals with improving the thermoelectric properties of the material. In this work, a method of improving the efficiency of the TEG by increasing the temperature difference is proposed. To accomplish this, a lens is used to concentrate solar radiation on the membrane of the TEG. By focusing solar radiation, the input heat flux increases; the temperature difference also increases; and the efficiency of the TEG improves as well.

Two implementations of the TEG are explored. The first one involves a simple TEG implementation using a glass substrate with p-type polysilicon and aluminum as the thermoelectric materials. Although a significant amount of heat is lost through the substrate, test results still demonstrate that a significant improvement in the device efficiency as the input heat flux is increased. The second implementation involves fabricating the TEG on a SOI substrate where the buried oxide layer is not etched and a thin portion of the handle layer is retained to provide additional structural stability. The thermoelectric materials for this TEG implementation are p-type silicon and aluminum. Although this implementation performs poorly than when both handle and buried oxide layers of the SOI under the membrane and thermoelements are etched, a SOI wafer with a thinner device layer is used to compensate for the losses.

The fabricated TEGs are characterized using a laser test set-up where the input power is varied up to 1 W and the spot size diameter is fixed at 1 mm. Measurement results on fabricated TEGs with 1 W input power exhibited a temperature difference of up to 226?C, open-circuit voltage of 3 V, output power of 25 ?W, and about 10 times improvement in conversion efficiency. The fabricated TEGs are also tested using a solar simulator and three lenses of different diameters to emulate conditions where the device would be deployed as a solar TEG. Using a 50.8 mm diameter lens, the largest temperature difference measured is 18?C, which gives an open-circuit voltage and output power of 803 mV and 431 nW, respectively.

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